With the London marathon taking place tomorrow, this story seems the most appropriate to look at today.
Shock horror, it seems that not all fitness trackers can accurately monitor the distance their wearers are running, according to a study reported on the BBC news website.
Quite apart from the difference in stride length for individual wearers - and I really hope they took that into account - trying to monitor the distance on a treadmill as opposed to via GPS seems distinctly risky, as there is no guarantee that the treadmill is entirely accurate.
Even using GPS may not be infallible - the first 5k Race for Life I took part in was apparently only 4.8km, according to the GPS on Runkeeper. I like to think that the organisers wouldn't have varied it by that much, and it certainly felt like at least 5km, if not longer!
In the end, does it really matter that much? I can't help thinking that the important thing from a fitness point of view is probably the time spent raising one's heart rate, and if your tracker doesn't have a heart rate monitor, then it probably does have a time-elapsed tracker. Worst case scenario, you can always look at your watch to see how long you've been out running/cycling/skipping....
And of course, go Viv Frost! Here's the link to her JustGiving page in case you want to sponsor her.